This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Neurotransmitter:sodium symporters (NSS) regulate neurotransmission at the synaptic cleft through a reuptake mechanism. In this protein family, monoamine transporters including dopamine transporter (DAT), serotonin transporter (SERT) and norepinephrine transporter (NET), are primary targets for antidepressants and psychostimulants. Although the pharmacological effects of these drugs have been studied extensively, little structural information is available, thereby hampering the thorough understanding of the inhibition mechanism and hindering structure based drug design. My current research is focused on studying atomic structures of antidepressants binding sites in neurotransmitter transporters using bacterial homologues. By applying a drug screening strategy called scintillation proximity assay (SPA), We identified a group of drugs that have binding affinity to bacterial transporters. The first drug target desipramine, a tricyclic antidepressant, has been cocrystallized with LeuT, a bacterial amino acid transporter. Homology models of human proteins were built by threading their sequences onto the LeuT-desipramine structure. I further designed both loss-of-function mutations in human SERT and gain-of-function mutations in human DAT to provide strong evidence supporting that the drug binding pockets are probably conserved in human neurotransmitter transporters. The other drug candidates are being test. This work will reveal how different antidepressants block neurotransmitter transporters, prevent conformational changes, and therefore inhibit substrate transport.